Computers include general purpose central processing units (CPUs) that are designed to execute a specific set of system instructions. A group of processors that have similar architecture or design specifications may be considered to be members of the same processor family. Examples of current processor families include the Motorola 680×0 processor family, manufactured by Motorola, Inc. of Phoenix, Ariz.; the Intel 80×86 processor family, manufactured by Intel Corporation of Sunnyvale, Calif.; and the PowerPC processor family, which is manufactured by Motorola, Inc. and used in computers manufactured by Apple Computer, Inc. of Cupertino, Calif. Although a group of processors may be in the same family because of their similar architecture and design considerations, processors may vary widely within a family according to their clock speed and other performance parameters.
Each family of microprocessors executes instructions that are unique to the processor family. The collective set of instructions that a processor or family of processors can execute is known as the processor's instruction set. As an example, the instruction set used by the Intel 80X86 processor family is incompatible with the instruction set used by the PowerPC processor family. The Intel 80X86 instruction set is based on the Complex Instruction Set Computer (CISC) format. The Motorola PowerPC instruction set is based on the Reduced Instruction Set Computer (RISC) format. CISC processors use a large number of instructions, some of which can perform rather complicated functions, but which require generally many clock cycles to execute. RISC processors use a smaller number of available instructions to perform a simpler set of functions that are executed at a much higher rate.
The uniqueness of the processor family among computer systems also typically results in incompatibility among the other elements of hardware architecture of the computer systems. A computer system manufactured with a processor from the Intel 80X86 processor family will have a hardware architecture that is different from the hardware architecture of a computer system manufactured with a processor from the PowerPC processor family. Because of the uniqueness of the processor instruction set and a computer system's hardware architecture, application software programs are typically written to run on a particular computer system running a particular operating system.
A computer manufacturer will want to maximize its market share by having more rather than fewer applications run on the microprocessor family associated with the computer manufacturer's product line. To expand the number of operating systems and application programs that can run on a computer system, a field of technology has developed in which a given computer having one type of CPU, called a host, will include an emulator program that allows the host computer to emulate the instructions of an unrelated type of CPU, called a guest. Thus, the host computer will execute an application that will cause one or more host instructions to be called in response to a given guest instruction. Thus, the host computer can both run software design for its own hardware architecture and software written for computers having an unrelated hardware architecture. As a more specific example, a computer system manufactured by Apple Computer, for example, may run operating systems and program written for PC-based computer systems. It may also be possible to use an emulator program to operate concurrently on a single CPU multiple incompatible operating systems. In this arrangement, although each operating system is incompatible with the other, an emulator program can host one of the two operating systems, allowing the otherwise incompatible operating systems to run concurrently on the same computer system.
When a guest computer system is emulated on a host computer system, the guest computer system is said to be a virtual machine, as the host computer system exists only as a software representation of the operation of the hardware architecture of the guest computer system. The terms emulator and virtual machine are sometimes used interchangeably to denote the ability to mimic or emulate the hardware architecture of an entire computer system. As an example, the Virtual PC software created by Connectix Corporation of San Mateo, Calif. emulates an entire computer that includes an Intel 80X86 Pentium processor and various motherboard components and cards. The operation of these components is emulated in the virtual machine that is being run on the host machine. An emulator program executing on the operating system software and hardware architecture of the host computer, such as a computer system having a PowerPC processor, mimics the operation of the entire guest computer system. The emulator program acts as the interchange between the hardware architecture of the host machine and the instructions transmitted by the software running within the emulated environment.
Over the years, the number of operating systems able to execute on a given processor family has increased markedly. For example, at present, the Windows operating system alone has several versions, such as Windows 3.1, Windows 95, Windows 98, Windows 98SE, Windows 2000, Windows NT, and Windows Millennium. Thus, a user of a computer system having an Intel 80X86 processor architecture may choose among several operating systems. Similarly, if the hardware architecture of a computer system running an Intel 80X86 architecture is being emulated in a host computer system, it may be desirable to run several virtual machines simultaneously, with each virtual machine operating according to a different operating system. As an example, it may be desirable to emulate on a Macintosh computer system several guest computers systems, each running a separate version of Windows. By doing so, the Macintosh user can take advantage of the wide variety of software applications designed to run on PC computer systems.
As the user increases the number of software applications and operating systems that is running on a computer system, it becomes more difficult for the user to manage the various tasks performed on the computer system. Accordingly, as the user increases the number of virtual machines running on the computer system, it becomes more difficult for the user to keep track of both the virtual machines and the respective applications running on the computer system's native operating system and the various virtual machines. For example, in operating systems that use windowing environments, such as Windows, the Macintosh operating system, and OS/2, for example, users may resize or move windows that correspond to various applications. Windows that are minimized or suspended are typically moved to a task bar, application bar or application list where they can be later accessed. Unfortunately, these application bars or lists generally do not present the user with enough information about the particular applications contained therein to allow the user to quickly find the particular application the user is interested in resuming. This problem is exacerbated as the number of applications and corresponding windows increases.